Mr. Watkins on Thermo-electricity* 305 



and \\ inch broad, the coils being well insulated by brown 

 paper and silk. 



We gather from these observations that Cav. Antinori em- 

 ployed an elongated coil as his electro-dynamic helix with 

 temporary magnets for eliciting the spark, while Professor 

 Wheatstone very judiciously resorted to the flat copper ribbon 

 coil, contrived and recommended for developing electricity 

 of feeble intensity by Professor Joseph Henry, of New Jersey 

 College, Princeton. 



Cav. Antinori gives different lengths of his coils, and I 

 presume in all cases that he had the advantage of the in- 

 fluence of temporary magnets, for I have experimented with 

 short and slender coils surrounding different metals, wood, 

 &c, and failed entirely in obtaining sparks under these con- 

 ditions. When the same coils enveloped soft iron, then I got a 

 spark, even a feeble one, from a coil the wire of which was 

 only seven feet long and ¥ \j of an inch in diameter. 



With Professor Henry's flat coil I always show larger 

 sparks than with an elongated wire coil and large temporary 

 magnet, and the snapping noise of the spark is certainly more 

 discernible. Hence I feel warranted in advising those who 

 desire to show the thermo-electric spark in its fullest effect 

 to use the flat ribbon coil of Professor Henry, in preference 

 to the elongated wire coil and temporary magnet, at the same 

 time permit me to add that those who possess a fair-sized 

 electro-magnet can exhibit the spark of a thermo-pile with 

 tolerable efficiency. 



My first attempt to repeat Professor Wheatstone's experi- 

 ments was with a very small and slender thermo-pile of thirty 

 pairs of elements three inches long, and a Henry's coil, and I 

 fully succeeded in eliciting the spark, A few days afterwards 

 in an interview I had with the Professor he was so kind as to 

 inform me that a thermo-electric pile of one inch square 

 plates in his possession had afforded him sparks of an in- 

 creased size to those obtained from the small pile described 

 in his communication to you. Hence I adopted the sugges- 

 tion of employing large metallic elements. 



I have recently made many experiments with thermo-piles of 

 various-sized and different-formed metallic elements, and dif- 

 ferent numbers of alternations, and find that the powers ex- 

 erted by the metallic elements of a pile are referrible to the 

 same law which governs the development of electricity de- 

 rived from other sources of excitation, namely, that quantity 

 is increased by mass. 



The spark has hitherto been obtained from the surface of 

 mercury, a metal at all times to be avoided in an apparatus 



Third Series. Vol. 11. No. 67. Sept. 1837. 2 R 



